/Engine/Source/Runtime/Core/Public/Math/UnrealMathUtility.h
FVector2D version of ClosestPointOnSegment. Returns closest point on a segment to a given 2D point. The idea is to project point on line formed by segment. Then we see if the closest point on the line is outside of segment or inside.
point on the segment defined by (StartPoint, EndPoint) that is closest to Point.
static FVector2D ClosestPointOnSegment2D ( const FVector2D & Point, const FVector2D & StartPoint, const FVector2D & EndPoint )